1. Field of the Invention
The present invention relates to a spline bearing.
2. Description of the Prior Art
A ball spline is shown in FIGS. 1 through 3 as an example of a spline bearing of the prior art.
In the drawings, six linear track grooves 1a are formed mutually in parallel along the lengthwise direction in spline shaft 1, and outer cylinder 2 is fit loosely on said spline shaft 1. An identical number as the number of above-mentioned track grooves 1a, namely six, of rolling element circulating paths (to be later described in detail) are formed corresponding to each of track grooves 1a in this outer cylinder 2. A large number of rolling elements in the form of balls 3 are arranged and contained within these rolling element circulating paths. Each ball 3 circulates by rolling over the above-mentioned track groove 1a accompanying relative motion of spline shaft 1 and outer cylinder 2, thereby bearing the load between spline shaft 1 and outer cylinder 2.
Outer cylinder 2 has cylindrical outer cylinder body 5, a pair of end caps 6 each formed into a circular shape and coupled to both ends of said outer cylinder body 5 by fitting inside, and seals 7 attached to the outside surfaces of both of said end caps 6. However, only the end cap 6 and seal 7 on one side are shown in the drawings (FIG. 1).
As is clear from FIGS. 1 and 2, the rolling element circulating paths described above are composed of load bearing track grooves 9a, namely load bearing tracks, and return paths 9b, each formed linearly and mutually in parallel in outer cylinder 5, and pairs of semi-circular direction changing paths 9c (see FIG. 1) which connect these at both ends. Furthermore, the above-mentioned load bearing track grooves 9a oppose track grooves 1a of spline shaft 1.
Ball splines having the above-mentioned constitution are used, for example, in mechanical portions to be made to perform linear relative motion while simultaneously bearing a radial load and torque such as in industrial robots and transfer machines.
In the case of apparatuses requiring an extremely high degree of operating accuracy as in the manner of industrial robots and so forth, it is necessary to detect the relative positions of spline shaft 1 and outer cylinder 2 with high precision. A device composed of long linear scale 11 and sensor 12 as shown in FIG. 1 are additionally provided as a position detection device for this purpose. This linear scale 11 is, for example, magnetized with a large number of magnetic poles minutely arranged in its lengthwise direction. In the case of, for example, using spline shaft 1 for the fixed side, linear scale 11 is fixed with respect to said spline shaft 1. In addition, since sensor 12 is composed of a magnetic sensor, it is mounted on outer cylinder 2.
As is clear from the above, in the spline bearing of the prior aft, the amount of space occupied by the parts that compose the position detection device provided to detect the relative positions of the spline shaft and outer cylinder is large, while said position detection device is also relatively expensive. Thus, this results in a problem that must be solved in terms of attempting to reduce the size and lower the cost of an industrial robot and so forth in which it is to be incorporated.
In addition, since the above-mentioned spline bearing is mounted by means of brackets and so forth on a prescribed base member (not shown), and linear scale 11 is also mounted to said base member by means of brackets and so forth, a large number of members, including this base member, brackets and so forth, are mechanically juxtaposed between spline shaft 1 and linear scale 11. Accordingly, even if it is attempted to set the detection accuracy achieved with the above-mentioned position detection device at a high level, it was not always easy to increase the reliability of relative positioning of spline shaft 1 and outer cylinder 2 due to the effects of the mounting errors and so forth between each of these members.